This invention relates generally to geometric configurations used in the construction of gas bearing X-Y-.theta. stage assemblies commonly utilized to facilitate the movement of an object along precisely orthogonal X and Y axes of motion and rotationally along a .theta.-axis about a Z direction which is orthogonal to the X and Y axes. One typical use for such gas bearing X-Y-.theta. stage assemblies is to step and position a semiconductive wafer along X, Y and .theta. coordinates under the imaging lens of a step-and-repeat camera. Thus, different regions of the semiconductive wafer can be sequentially aligned and exposed in accordance with a pattern of a reticle. A typical prior art gas bearing X-Y stage assembly is shown in U.S. Pat. No. 3,722,996 entitled OPTICAL PATTERN GENERATOR OR REPEATING PROJECTOR OR THE LIKE and issued Mar. 27, 1973, to Wayne L. Fox. Disclosed in that patent is an X-Y stage which is located directly with respect to a top reference surface of a base by three stem supported bearings. It is guided along a reference edge surface of a generally L-shaped frame extending across the top reference surface of the base and orthogonal to a reference edge surface of the base by a pair of guideway bearings attached to the X-Y stage. The L-shaped frame is guided along the reference edge surface of the base by another pair of guideway bearings attached to the L-shaped frame. The orthogonality of the resulting X and Y axes of motion of the X-Y stage of that patent is determined by the accuracy of the orthogonal relationship of the reference edge surface of the L-shaped frame and the guideway bearings attached to the L-shaped frame that are used to guide the L-shaped frame along the reference edge surface of the base. Although no .theta.-axis rotational capability is disclosed in that patent, .theta.-axis stages are employed with such X-Y stage assemblies to construct an X-Y-.theta. stage assembly, which assemblies typically comprise a .dwnarw.-axis stage mounted on top of such an X-Y stage.
The gas bearing X-Y stage assembly of U.S. Pat. No. 3,722,996 has the disadvantage that the gas bearings used as guideway bearings for guiding the X-Y stage and the L-shaped frame along their corresponding reference surfaces must be of the vacuum compensated type in order to force these gas bearings into juxtaposition with their corresponding reference surfaces. Thus, if the vacuum supply pressure varies, the flying heights of the gas bearings vary concomitantly because the load on the gas bearings is proportional to the vacuum supply pressure and the flying height is a function of the load. Should the vacuum supply pressure drop to zero there would be no load at all and the elements of the gas bearing X-Y stage assembly would simply drift apart. What would be desirable is an improved multi-axis gas bearing stage assembly wherein the load impressed upon all of the guideway bearings is proportional to the gas supply pressure used to activate the guideway bearings themselves rather than a function of the vacuum supply pressure.
Another disadvantage of that gas bearing X-Y stage assembly is its inefficient use of space. As illustrated in U.S. Pat. No. 3,722,996, the size of the envelope encompassing the X-Y stage and its extent of travel along the X and Y axes of motion is only a fraction of the total lateral area of the gas bearing X-Y stage assembly. What would be desirable is an improved multi-axis gas bearing stage assembly wherein the size of the envelope encompassing an X-Y-.theta. stage and its extent of travel along the X, Y and .theta. axes of motion is substantially the same as the total lateral area of the multi-axis gas bearing stage assembly.
Another disadvantage of the gas bearing X-Y stage assembly of U.S. Pat. No. 3,722,996 and virtually all other gas bearing X-Y stage assemblies is that the orthogonality of its X and Y axes of motion is solely determined by the mechanical orthogonality of a reference surface (the reference edge surface of the L-shaped frame) and guideway bearings (the two guideway bearings attached to the L-shaped frame used for guiding the L-shaped member along the reference edge surface of the base) of an intermediate stage element (the L-shaped frame). What would be desirable is an improved multi-axis gas bearing stage assembly wherein the orthogonality of its X and Y axes of motion is controlled with reference to a three dimensional measurement system, whose frame of reference is the base of the multi-axis gas bearing stage assembly, and which may in turn be correlated with a measurement standard of orthogonality whenever desired. If the measurment standard of orthogonality were also a component sub-assembly of the multi-axis gas bearing stage assembly then control of the orthogonality of its X and Y axes of motion could be accomplished as a software function.
Further, it would be desirable to have an improved multi-axis gas bearing stage assembly wherein the motion control of the Y-axis is bifurcated by having parallel but separated Y and Y motion controls that are independently controllable for rotating the X-Y-.theta. stage in a preselected manner. Thus, a semiconductive wafer positioned on a wafer chuck mounted upon the X-Y-.theta. stage could be oriented rotationally with respect to the base of a step-and-repeat camera without employing a .theta.-axis stage between the X-Y stage and the wafer chuck.
Still another disadvantage of some gas bearing X-Y-.theta. stage assemblies utilized for semiconductive wafers is that while they must rotate the semiconductive wafer in pitch and roll during wafer processing, their metrology systems do not directly measure the position of the semiconductive wafer. Rather, their metrology systems measure the position of an X-Y-.theta. stage that either supports, or is supported by, a leveling mechanism and those measurements are made in an offset manner. The resulting position of the semiconductive wafer is then subject to an error equal to the product of an effective offsetting distance and the pitch or roll angle. This type of measurement error is called Abbe offset error (after the German optical physicist Ernst Abbe). An example of an X-Y-.theta. stage assembly subject to such an Abbe offset error is shown in U.S. Pat. No. 4,391,494 entitled APPARATUS FOR PROJECTING A SERIES OF IMAGES ONTO DIES OF A SEMICONDUCTOR WAFER and issued July 5, 1983 to Ronald S. Hershel.
Accordingly, it is a principal object of this invention to provide an improved multi-axis gas bearing stage assembly wherein the size of the lateral envelope encompassing an X-Y-.theta. stage and its extent of travel along the X and Y axes of motion is substantially the same as the total lateral area of the improved multi-axis gas bearing stage assembly.
Another object of this invention is to provide an improved multi-axis gas bearing stage assembly wherein the orthogonality of its X and Y axes of motion is controlled with reference to a three dimensional measurement system whose frame of reference is the base of the improved multi-axis gas bearing stage assembly.
Another object of this invention is to provide a three dimensional measurement system for use as a reference for the control of the orthogonality of the X and Y axes of motion of the improved multi-axis gas bearing stage assembly.
Another object of this invention is to provide a measurement standard of orthogonality, which is a component sub-assembly of the improved multi-axis gas bearing stage assembly, for use in correlating the three dimensional measurement system.
Another object of this invention is to provide a method of correlating the three dimensional measurement system with the measurement standard of orthogonality.
Another object of this invention is to provide an improved multi-axis gas bearing stage assembly wherein the motion control of the Y-axis is bifurcated by having parallel but separated Y.sub.1 and Y.sub.2 motion controls that are independently controllable, for use in rotatably positioning the X-Y-.theta. stage of the improved multi-axis stage assembly along a .theta.-axis of motion.
Another object of this invention is to provide a method of controlling the rotational positioning of the X-Y-.theta. stage of the improved multi-axis gas bearing stage assembly along the .theta.-axis of motion.
Another object of this invention is to provide an improved multi-axis stage assembly wherein a semiconductive wafer loading plane of the improved multi-axis stage assembly is fixedly located relative to a utilization apparatus in the directions of the X, Y and .theta. axes of motion of the X-Y-.theta. stage, for eliminating Abbe offset measurement errors to the plane of the semiconductive wafer.
Another object of this invention is to provide an improved multi-axis stage assembly wherein the motions of a chuck mounting spider (and therefore, of a wafer chuck of the X-Y-.theta. stage of the improved multi-axis stage assembly) are directly controlled by a three axis interferometer system along the X, Y and .theta. axes of motion, for minimizing various measurement errors, including other Abbe offset measurement errors to the position of the semiconductive wafer.
Another object of this invention is to provide an improved multi-axis stage assembly wherein the motions of the chuck mounting spider are directly controlled by a five axis interferometer system along the X, Y and .theta. axes of motion, for eliminating still other Abbe offset measurement errors, to the position of the semiconductive wafer, in the pitch and roll directions.
Still another object of this invention is to provide an improved multi-axis gas bearing stage assembly wherein the X-Y-.theta. stage is located by guideway bearings whose loading is proportional to the gas supply pressure used to activate the guideway bearings themselves.
These and other objects, which will become apparent from an inspection of the accompanying drawings and a reading of the associated description, are accomplished according to illustrated preferred embodiments of the present invention by providing improved multi-axis gas bearing stage assemblies with an intermediate stage element that is guided along a reference edge surface of a base by a single guideway bearing, and controlled with reference to a two dimensional measurement sub-system, of a three dimensional measurement system, whose frame of reference is the base of the multi-axis gas bearing stage assembly. An X-Y-.theta. stage is located with respect to the intermediate stage element by a plurality of gas bearings. It is guided along a reference edge surface of the intermediate stage element by a pair of guideway bearings, and controlled with reference to a third dimension measurement sub-system of the three dimensional measurement system, whose frame of reference is the intermediate stage element. All three of the guideway bearings are forced into juxtaposition with their corresponding reference surfaces by opposing axially loaded guideway bearing assemblies which are loaded in a hysteresis free manner by the gas supply pressure.
The improved multi-axis gas bearing stage assemblies also include a measurement standard of orthogonality comprising a two dimensional pattern of indicia mounted upon the X-Y-.theta. stage and an optical sub-system located with respect to the base sub-assembly for viewing the indicia. The two dimensional pattern of indicia is sequentially aligned with respect to the optical sub-system and a corresponding software correction is made to the two dimensional measurement sub-system used as a reference for the control of the intermediate stage element.
Differential motion control of the intermediate stage element, with reference to its two dimensional measurement sub-system, is utilized to provide rotational positioning of the X-Y-.theta. stage along a .theta.-axis of motion about a direction orthogonal to the X and Y axes of motion. The method of controlling the rotational positioning of the intermediate stage element and the X-Y position of the X-Y-.theta. stage comprises utilizing the separation of the two portions of the two dimensional measuring sub-system and a tangent function of the preselected rotational angle to calculate the .DELTA.Y length difference of the two portions of the two dimensional measuring sub-system; calculating the X-axis position by utilizing a cosine function of the rotational angle .theta.; and calculating the Y-axis position by utilizing a first portion of the two dimensional measurement sub-system and a sine function of the rotational angle .theta..
Alternative preferred embodiments of the present invention utilize different methods of eliminating various manifestations of Abbe offset errors. In one preferred embodiment, the base of the multi-axis gas bearing stage assembly is located with respect to an optical imaging system of a step-and-repeat camera by a kinematic positioning apparatus wherein three kinematic constraints are coplaner with the semiconductive wafer being processed. In another preferred embodiment, the X-Y position of the semiconductive wafer plane is controlled by software computation. In another preferred embodiment, a three axis interferometer measuring system controls the position of a chuck mounting spider (which supports a wafer chuck of the X-Y-.theta. stage) directly. The chuck mounting spider's position is measured in a plane parallel to and nearly coplanar with a semiconductive wafer being processed, and the semiconductive wafer's actual position is determined by computer calculation. In still another preferred embodiment, a five axis interferometer measuring system is used to provide five axis control of the position of the chuck mounting spider. Thus, remaining minute pitch and roll derived Abbe offset measurement errors (of the position of the semiconductive wafer being processed) are eliminated.